Cyclic acid-base bifunctional reaction

Figure 2.45 Acid-base bifunctional reaction mechanism in the synthesis of cyclic carbonates over mixed oxides obtained from a Mg-Al hydrotalcite. Source Kaneda et al. [28].

In this solvent the reaction is catalyzed by small amounts of trimethyl-amine and especially pyridine (cf. 9). The same effect occurs in the reaction of iV -methylaniline with 2-iV -methylanilino-4,6-dichloro-s-triazine. In benzene solution, the amine hydrochloride is so insoluble that the reaction could be followed by recovery. of the salt. However, this precluded study mider Bitter and Zollinger s conditions of catalysis by strong mineral acids in the sense of Banks (acid-base pre-equilibrium in solution). Instead, a new catalytic effect was revealed when the influence of organic acids was tested. This was assumed to depend on the bifunctional character of these catalysts, which act as both a proton donor and an acceptor in the transition state. In striking agreement with this conclusion, a-pyridone is very reactive and o-nitrophenol is not. Furthermore, since neither y-pyridone nor -nitrophenol are active, the structure of the catalyst must meet the conformational requirements for a cyclic transition state. Probably a concerted process involving structure 10 in the rate-determining step... 

Among all the possible approach to polysubstimted cyclohexane derivatives, Dixon et al. proposed a further and very elegant alternative approach wherein the asymmetric organocatalytic cascade reaction reties on the synergic action of a bifunctional base/Br0nsted acid catalyst 158 and a cyclic secondary amine (5)-76 (Scheme 2.49) [77]. 

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